1. Field of the Invention
The present invention relates to an isostatic molding method for ceramic powders, metal powders and their composites, more precisely, to a semiconductor package for which is used high-melting-point metal powders of W, Mo and the like, composites such as Cu/W, Cu/Mo, W/Ni/Cu, W/Ni/Fe and the like, or composites such as Mo/TiC, Al/SiC and the like, and also to an isostatic molding method for producing heat-radiating substrates for semiconductor packages.
2. Description of the Related Art
Heat radiation from packages of semiconductor devices that are used in communication-related appliances such as portable telephones and others is the recent important theme in the art. Heat-radiating members on which those semiconductor devices for microwaves are to be mounted are being in demand. For the heat-radiating members, metallic materials of aluminum, copper and the like may be taken into consideration in view of their heat conductivity. However, as greatly expanding with heat, members of such metallic materials are often problematic in that, when bonded to semiconductor chips of silicon and the like or to insulating members such as aluminum nitride substrates and the like mounted with silicon, they may be deformed or cracked due to the heat change in solder bonding or repeated use and due to the difference in thermal expansion between the metallic members and the semiconductor or insulating members. Therefore, materials with good heat conductivity, of which the thermal expansion approaches to that of semiconductors and insulating ceramic materials, are desired.
Aluminum nitride to be mounted with semiconductor chips is generally lined with a Cu sheet on its back surface.
As heat-radiating substrates meeting the requirements noted above, composite materials of tungsten (W)-copper (Cu) (hereinafter referred to as W—Cu composites) have been proposed.
To produce such W—Cu composites, employed is a method which is as follows: W powder is molded under compression to give a green compact. The green compact is then sintered in a reducing atmosphere into a porous body of W having a predetermined degree of porosity. Next, copper is infiltrated into the porous body in a reducing atmosphere at a temperature not lower than the melting point of copper to obtain a W—Cu composite.
In order to evade the problem of thermal strain noted above, heat-radiating substrates for IC (integrated circuit) packages, for which are used ceramic materials, must be so designed that their thermal expansion approaches to that of alumina, beryllia and the like. For these, used are W—Cu composites with from 10 to 15% by mass of copper infiltrated thereinto.
Those W—Cu composites well used for heat-radiating substrates are produced by infiltrating Cu into porous bodies of W. In general, they essentially have a Cu content of from 10 to 20% by mass, and have good characteristics. For example, they have a thermal expansion coefficient of from 7 to 8×10−6/K, and a thermal conductivity of from 180 to 200 W/m·K. However, in the recent tendency toward lightweight, thin and small parts in the art, the disadvantages of high density and heavy weight of W—Cu composites are being serious problems. In addition, since W—Cu composites are worked by cutting into products, their another drawback is that they could not be thinned well.
Specific methods for producing Mo—Cu green compacts are mentioned below.
One method is known for producing green compacts of ordinary ceramics, metal powders and their composites through isostatic molding, which comprises putting a powder to be molded, for example a powder of Mo or the like, into a rubber bag mold or the like, sealing the mold, then putting it into a hydraulic pressure tank filled with water, and applying an external hydraulic press to the rubber mold so as to press the powder into a green compact.
Also known is another method for producing such green compact through ordinary powder pressing, for which is used a pressing device. The pressing device comprises a mortar to give the inner wall surface for the cavity, and upper and lower rods to give the upper and lower surfaces for the cavity. Concretely, a powder of Mo or the like is filled into the cavity to be formed by the mortar and the rods, sealed with the upper rod, and then compressed by the upper and lower rods into a green compact.
Using conventional Cu—Mo composites for producing heat-radiating substrates for semiconductor packages for microwaves is problematic in various aspects of, for example, the characteristics of the composites, the workability thereof, and even the thickness of the products to be produced from them.
For producing heat-radiating substrates such as those noted above, intermediate products of green compacts for them are prepared. In conventional isostatic molding methods, green compacts having been pressed with a uniform pressure could be obtained. In those, however, since flexible rubber molds or the like are used, well-shaped plates or green compacts having a specific shape are difficult to obtain.
On the other hand, in powder pressing methods for producing large-sized green compacts by applying pressure to powder from the upper and lower sides, large pressure is applied to powder. Therefore, for the methods, the mold, especially the mortar to be used must be so designed that its mechanical strength is satisfactorily high. For these reasons, the methods are defective in that the costs for the mold are high.